The invention relates generally to a catalyst activator for heating the catalyst and conditioning it with a gas. The invention relates more particularly to a catalyst activator for conditioning olefin polymerization catalysts.
Many solid catalyst compositions, such as those employed in hydrocarbon conversion operations, e.g., polymerization, cracking, dehydrogenation, hydrogenation, and the like, are activated by subjecting the raw catalyst to elevated temperatures for an interval of time, while passing over the catalyst a stream of conditioning fluid which is inert, non-oxidizing, non-reducing, oxidizing, reducing, dry, or the like, depending on the particular nature of the catalyst and its intended use. One of the common objects of such treatment is the removal of moisture from the catalyst, since water is a catalyst poison in many applications.
Catalyst activation processes comprise drying an activating fluid such as air and passing it through a catalyst bed at a constant rate, while applying heat, until the catalyst reaches the desired temperature, at which point the catalyst is held at the activation temperature for the proper length of time. However, solid catalyst compositions are often relatively impermeable, thus requiring a shallow bed in order to obtain the required flow of activating fluid. The bed thus becomes large and expensive.
To alleviate the deficiencies encountered in activating solid particulate catalysts, including the continuous removal of impurities and catalyst poisons from the activation zones and temperature control of the bed, fluidized activation processes have been developed. In these processes, the catalyst is fluidized with a stream of activating fluid at elevated temperatures.
One type of fluidized bed catalyst activator has a grid plate through which fluidizing gas flows upwardly to levitate particulate matter, forming the fluidized bed. The upper surface of the grid plate is machined with an array of generally conical depressions that overlap essentially completely so the upper surface of the grid plate has no flat surfaces on which catalyst particles can accumulate and escape the conditioning effect of the fluidizing gas.
In one known catalyst activator the grid plate (see FIG. 1) is nominally 1.13 inch (28.58 mm) thick (dimension a, FIG. 1), has an inside diameter of 42 inches (1.07 m), and the generally conical depressions are predominantly 90-degree (angle b, FIG. 1) conical depressions having a nominal depth of one inch (25.4 mm) (dimensions a–c), a nominal diameter d at the upper major surface of 2.078 inches (52.78 mm), spaced apart in each direction by 1.781 inches (45.2 mm) from center to center, thus overlapping by about 16.7%.
The nominal layout of a single 90-degree conical depression of the 42-inch (1.07 m) grid plate is shown in FIG. 1. This known grid plate has 418 90-degree conical depressions, and around the outside periphery, where there is less clearance allowed to drill depressions, the grid plate has eighteen 70.5-degree conical depressions and twelve 60-degree conical depressions, for a total of 448 conical depressions of the three sizes provided. The depth of each type of conical depression is the same. The apex of each conical depression was bored through to the lower surface of the grid plate by drilling a 0.078 inch (1.98 mm) diameter (dimension e), 0.125 inch (3.175 mm) deep (dimension c) bore.
As polymerization reactors have increased in size or number at a given plant location, the amount of catalyst needed has increased, and a need has arisen to increase the amount of catalyst activated at a given time.
One approach to this problem is to provide more than one catalyst activator. This approach has the problem of requiring more equipment, operating personnel and other resources, including the vessels, sensors, piping, wiring, and computer capacity, than one catalyst activator.
Another approach to this problem is to operate a catalyst activator of the same diameter as before, but with a deeper fluidized bed of catalyst. A problem created by this approach is that a deeper fluidized bed allows less activation air and more activation effluent from the fluidized bed to contact each particle of the catalyst, which may reduce the quality of the resulting activated catalyst.
Yet another approach, increasing the diameter of the activator, has previously been rejected for at least two reasons.
First, increasing the diameter of the inner vessel reduces its surface area exposed to flue gases, as a proportion of the interior volume. As the diameter increases, the wall surface area of the inner vessel increases proportionally to the increase in diameter, while the volume of the inner vessel (assuming the depth of the vessel remains constant) increases proportionally to the square of the increase in diameter. Also, the heat must be transferred further to reach the center of a larger-diameter vessel. These effects reduce the amount of heat transferred per unit volume, per unit time, and per particle of the catalyst in the inner vessel.
Second, increasing the diameter of the grid plate that establishes a fluidized bed, and the size of the catalyst charge the grid plate is required to support, increases the weight of the grid plate, the diameter the grid plate must span, and the weight of catalyst the grid plate must bear. Simply scaling up the grid plate dimensions would require the grid plate to be quite thick to support both its own weight and that of a larger catalyst charge over a greater span. The depressions milled into the surface of the grid plate to eliminate flat areas further exacerbate this problem, as a considerable amount of metal is removed to form the depressions, thus effectively decreasing the thickness of the grid plate.
Thus, simply scaling up the catalyst activator is not a satisfactory solution to the problem of processing more catalyst per batch.